Geiger-Muller tube – a detector that measures the count-rate (pulse-rate) of a radioactive sample

(pronounced guy-ger muh-ler)

To remember the meaning of Geiger-Muller tube, use the following mnemonic:

The guys and girls pointed multiple tubes (Geiger-Muller tube) at the radioactive sample and the devices pulsed rapidly (pulse-rate).

A Geiger-Muller tube, often called a Geiger counter, is a device used to detect and measure ionising radiation such as alpha particles, beta particles and gamma rays. It consists of a sealed metal tube filled with low-pressure gas (usually argon) and contains two electrodes with a high voltage between them. 

When ionising radiation enters the tube through the thin window, it ionises the gas atoms inside, knocking electrons off them and creating ions. The charged particles are attracted to the electrode, creating a pulse of electric current that can be counted and displayed or heard as a clicking sound. Each click represents a particle of radiation being detected. These devices are portable and relatively inexpensive, which means they are highly used in any potentially radioactive environment to maintain a safe distance from any radioactive hotspots.